1. Field of the Invention
The present invention relates to a high-frequency module, and in particular, to a high-frequency switch module arranged to switch a communication signal having a specific frequency using a FET switch.
2. Description of the Related Art
Presently, there is a plurality of specifications used in wireless communications systems for cellular phones or other suitable devices. In Europe, for example, a multi-band GSM system is used. In the GSM system, a plurality of communication signals (transmission/reception signals) using different frequency bands, such as an 850 MHz band and a 900 MHz band, are used. Furthermore, an 1800 MHz band and a 1900 MHz band are also used. When these multiple communication signals in different frequency bands are received by a single antenna, communication signals other than those of a target frequency band are not processed. Even in the case of a single communication signal, a reception signal is not processed during transmission, and a transmission signal is not processed during reception. Thus, when transmission and reception are performed using a single antenna, there is a need for switching between a path for transmitting a transmission signal and a path for receiving a reception signal. For such switching, various high-speed switch modules using FET switches have been developed (see Japanese Unexamined Patent Application Publication No. 2002-185356, for example).
In Japanese Unexamined Patent Application Publication No. 2002-185356, the high-frequency switch module as illustrated in FIG. 9 is provided. FIG. 9 is a block diagram illustrating the configuration of an existing high-frequency switch module. The existing high-frequency switch module is provided with a FET switch SW100 including a transmission port RF101 to which the transmission signal (first transmission signal) of a first communication signal and the transmission signal (second transmission signal) of a second communication signal are input, a first reception port RF102 arranged to output the reception signal (first reception signal) of the first communication signal, a second reception port RF103 arranged to output the reception signal (second reception signal) of the second communication signal, an antenna port ANT0 arranged to perform input/output of the first and second transmission signals and of the first reception signal and the second reception signal, for an antenna ANT. A switch defined by a semiconductor device, preferably a FET, is used for the FET switch SW100, and currently, a GaAs switch is often used. In the existing high-frequency module, a low pass filter LPF201 for attenuating the harmonics of the first and second transmission signals is connected to the transmission port RF101, a band pass filter BPF301 for passing the fundamental frequency of the first reception signal is connected to the first reception port RF102, and a band pass filter BPE302 for passing the fundamental frequency of the second reception signal is connected to the second reception port RF103.
In the high-frequency switch module described above, a transmission signal is input to an input terminal Tx1 which is connected to the transmission port RF101 of the FET switch SW100 via the low pass filter LPF201. This transmission signal is usually input after being amplified by a power amplifier connected in a prior stage. At this time, the higher harmonics of the fundamental frequency f0 of the transmission signal generated during the amplification are also input together with the transmission signal having the fundamental frequency f0. Here, if the low pass filter LPF201 of the high-frequency switch module illustrated in FIG. 9 has been set so as to attenuate the higher harmonics, the higher harmonics of the transmission signal input to the FET switch SW100 can be suppressed. For instance, by configuring the low pass filter LPF201 using a low pass filter attenuating the second harmonic (2·d f0) of the fundamental frequency f0 and a low pass filter attenuating the third harmonic (3·f0) of the fundamental frequency f0, these second and third harmonics are suppressed.
However, when the FET switch SW100 is defined by a GaAs switch, upon input of a high-frequency transmission signal, harmonic distortion is generated in the FET switch SW100, whereby harmonics such as the second harmonic and the third harmonic are uniformly output to each port. At this time, the low pass filter LPF201 at the harmonic frequencies appears to be in an open state having nearly infinite impedance from the viewpoint of the transmission port RF101. Thus, the harmonics generated in the FET switch SW100 are completely reflected at the transmission port RF101 side end of the low pass filter LPF201 and input to the FET switch SW100. Consequently, by letting “X” denote the initial harmonics and “α” denote an increase in the harmonics due to the complete reflection, harmonics of “X+α” are unfavorably output from the antenna port ANT0.
To suppress these harmonics, a GaAs switch which is unlikely to generate harmonics is required. However, a GaAs switch that is unlikely to generate harmonics does not exist. Alternatively, if a switch circuit including a diode switch is used, harmonics are unlikely to be generated. However, at least two diodes are required for switching between transmission and reception for each communication signal, and circuits accompanying the diodes are also required. This prevents a decrease in the size of a high-frequency switch module. Furthermore, the use of multiple diode switches causes an increase in power consumption and a decrease in response time. In particular, this influence increases as the number of the FET switch ports increases.